The objective of this proposed research is to study the mechanisms which control fetal breathing movements in utero. The chronically prepared fetal lamb will be used for these studies. We will study the effects of chronic intrauterine hypoxia on the pattern of fetal breathing movements. Chronic fetal hypoxia will be induced by long term microembolization of the maternal placental circulation. In additional studies we will examine the role of fetal peripheral and central respiratory receptors in both respiratory acidosis (hypercapnia) and metabolic acidosis. These studies of the mechanisms for respiratory control will be investigated in both normoxic and hypoxic lambs in utero. These studies are aimed at establishing a further physiological basis for use of fetal breathing movements in evaluation of high risk pregnancies.